As is well known, it is now possible, with the use of a so-called sniperscope or heat camera, to detect and form an image of an object that emits infra-red radiation. The technology has been refined to such an extent that a clear thermal picture can be obtained at a substantial distance from an object, even if the temperature of the object exceeds that of its surroundings by only a few degrees. Equipment embodying this technology is finding military applications in, for example, installations comprising tank weapons and anti-tank defense weapons, wherein heat responsive devices are used for night-time weapon aiming.
The use of heat-responsive sighting equipment requires special training, because the image presented by such a device is of a different character than that perceived with visible light. Accordingly, it is necessary to afford extensive night-time practice to personnel who are expected to use such equipment, in order to develop their ability to detect targets and to recognize objects of the type at which they may have to fire in actual combat. For such training it is necessary to have targets which will emit heat radiation patterns that realistically simulate specific objects at which combat fire might be aimed, to enable realistic practice operations to be conducted at night.
One type of practice target heretofore proposed for this purpose comprised a metal plate having the outline of a tank or other object to be simulated, together with a burner of some kind that directly heated the plate to raise its temperature above that of its surroundings. A real object at which combat firing would be conducted ordinarily has a pattern of different temperatures, which influence the image of the object at a heat responsive device, but a heated plate tends to have a substantially uniform temperature across its entire surface and therefore its thermal image poorly simulates a real object. Such a heated-plate target was further unrealistic in that it tended to warm up and cool off rather slowly, and therefore it could not satisfactorily simulate a moving vehicle or the like which may appear and disappear rather abruptly. High energy consumption was also a disadvantage of such a target. Furthermore, the plate had to have substantial thickness in order not to be burned through by the heater, and therefore it was heavy and difficult to move.
In another and somewhat better proposal for a thermal target, the necessary elevated temperature was obtained by means of a resistance wire network that extended over the target, or over such portions of a target surface as corresponded to an object to be simulated, and this network was connected with a battery or other current source. The time required for heating and cooling was relatively short, the profile of an object to be simulated could be reproduced fairly well, and the device was relatively light in weight and easily moved. A serious disadvantage, however, was the excessive vulnerability of the target to weapon fire. A shot placed on the electrical resistance network tended to break a part of the circuit or all of it, so that no more than a few hits the target lost its resemblance to the object it was intended to simulate and became substantially useless for training purposes.